Electric unit for a pump-storage power plant

ABSTRACT

A pumped-storage power plant is disclosed, such as an electric unit having a converter and a rotating electric synchronous machine. The converter is designed as a modular multilevel converter and the machine is directly connectable to the converter, wherein the converter has an adjustable voltage.

TECHNICAL FIELD

The invention relates to a pumped-storage power plant, in particular toan electric unit therefor, comprising a converter and a rotatingelectric synchronous machine, wherein the machine is provided in acavern.

PRIOR ART

Regenerative energy sources such as, for example, wind energy and solarenergy provide a continuously increasing proportion of the electricitydemand. These energy sources do have discontinuous operating times,however. Therefore, a direct and permanent supply of electricity toconsumers from these energy sources cannot be ensured. For this, energystores need to be used which enable rapid changes between a surplus ofelectricity and a deficit of electricity and whose power and energy flowdirection can be changed quickly and continuously.

In this case, there are different system available as energy storeswhich are in each case particularly suitable for specific quantities ofenergy and application cases. For low quantities of energy up toapproximately 20 MWh, kinetic stores (for example flywheels),electrochemical stores (batteries, redox flow cells) or electromagneticstores (capacitors, supercapacitors, superconducting coils) arepreferably used, depending on the application. For medium quantities ofenergy of up to a few 100 MWh, in principle thermodynamic stores(compressed-air stores, electrothermic stores) are particularly wellsuited. For large quantities of energy of typically over 100 MWh andusually over 1 GWh, pumped stores are used.

Pumped stores or pumped-storage power plants are of particular interestowing to the large amount of energy that can be stored. In this case,with surplus electricity water is pumped from a first natural storagebasin or storage basin set up artificially for this purpose into asecond storage basin positioned higher. In the process, the electricalenergy is converted into potential energy. In order to recoverelectricity, water is directed from the higher storage basin via aturbine back into the lower storage basin. For this system, minimizationof the losses in the conversion processes is particularly important.

Modern pumped stores have variable-speed drives. By decoupling the speedof the machines from a grid frequency, rotational speeds of the pumpsand turbines can be set such that they are operated close to optimumefficiency. In addition, the variation in the speed during pumpoperation makes it possible to freely adjust the power consumption. Inparticular, systems with a variable speed can be connected to orsynchronized with the grid quickly from a standstill.

Pumped stores in accordance with the prior art have double-fedasynchronous machines and power electronics frequency converters,whereby speed regulation of a pump and a turbine is possible. In thiscase, a stator of the double-fed asynchronous machine is connecteddirectly to the electric grid with a grid frequency. A rotor of thedouble-fed asynchronous machine is connected to the grid via a frequencytransformer and can therefore have a variable frequency. Thus, firstly apump power is regulated and secondly the efficiency of the system can beincreased, if required.

In the case of pumped stores, provision is often made for the machineequipment such as, for example, turbines, pumps and motor generators, tobe accommodated in a cavern introduced into rock, in a cavity or in aclosed area beneath the second storage basin, for example. Another formof pumped store provides pump, turbine and motor generator at the lowerend of a shaft, wherein further power plant components are providedabove ground in a building or likewise in the shaft, for example.

This way of accommodating the machine equipment is a consequence of thecircumstance whereby the turbines need to be positioned sufficientlybelow the lower storage basin in order to avoid cavitation. For spaceand geometry reasons, this is usually only possible by virtue of apowerhouse for accommodating the machine equipment being builtunderground. Furthermore, an underground design provides protectionagainst environmental influences such as descents of avalanches, forexample. Furthermore, for environmental protection or landscapeprotection reasons, an underground design is likewise often used.

The converters according to the prior art used in this connectiontypically have an output voltage of 3 to 6 kV. In contrast, in the caseof the electric machine, a voltage of 10 to 21 kV is usually used.Therefore, it is necessary to provide a power transformer between anoutput of the converter and terminals of the machine, which powertransformer bypasses the respective voltage differences. The highcurrents in this case require that the transformer is provided in thedirect vicinity of the machine, that is to say within the cavern.

An essential disadvantage of this arrangement with a double-fedasynchronous machine is an increased requirement on space within thecavern, as a result of which costs for erecting the pump-storage powerplant and digging the cavern are significantly increased. Moreover, thetransformer has losses of the order of 1-2% of the transferred activepower. Overall, the use of a transformer is disadvantageous owing to thehigh costs of the transformer. Furthermore, a transformer has aconsiderable risk of explosion owing to transformer oils and thereforerepresents a considerable safety risk.

A further disadvantage of converters according to the prior art is thatthe voltage difference is overcome in one or two stages or steps. As aresult of this, firstly, the high and steep voltage steps requiresmoothing or filtering in order not to overload an electrical isolationof the transformers which are directly coupled to the frequencyconverter, and the machine, and to avoid damage to the isolation. Asmoothing or filtering arrangement is expensive, causes further lossesof the active power and requires lots of space. Secondly, thetransferred power has a large harmonic component which harmonics causeadditional losses in the machine and in the connected transformers.

Against this background, the present invention is based on the object ofsimplifying the erection, operation and maintenance of a pumped-storagepower plant and increasing the efficiency.

DESCRIPTION OF THE INVENTION

This object is achieved by an electric unit for a pumped-storage powerplant as claimed in claim 1. Further advantageous configurations resultfrom the dependent claims, wherein back-references in the claims do notexclude any other expedient combinations of claims.

The invention relates to an electric unit for a pumped-storage powerplant, wherein said electric unit is connectable to an electric grid.The electric unit in this case comprises at least one frequencyconverter and a rotating electric synchronous machine, which acts asmotor or generator depending on a type of operation of the machine. Themachine is mechanically connectable, for example, to a water turbine anda water pump or a reversible pump turbine. The converter is designed inthis case as a modular multilevel converter MMC and is directlyconnected to the machine, that is to say without an intermediatelyconnected transformer, wherein the converter has an adjustable voltage.

In an advantageous configuration of the invention, at least the electricmachine is provided in a cavern, in a shaft or in a closed area beneaththe tailwater level.

In another advantageous embodiment, the converter has a multiplicity ofseries-connected unit cells for scaling an output voltage. The unitcells each have, for example, a capacitor or an inductor. Furthermore,the unit cells are separately controllable, for example for adapting theoutput voltage to the electric machine.

In a particularly advantageous embodiment, provision is made that edgesteepness and amplitude of pulses between an output of the converter andthe machine, for example terminals of the machine, is limitable torelieve the isolation.

In another advantageous embodiment, a multiplicity of voltage steps, forexample small steps, can be used to modify the output voltage.

Another embodiment provides that the output voltage has low harmonicdistortion, such that the grid connection conditions are fulfilledwithout additional filtering and no differences emerge in the case ofconfiguration of the machine with respect to harmonics compared to adirect connection to the grid. Such grid connection conditions aredefined, for example, in a grid code.

In one embodiment of the invention, provision is made that the converteris connected on the grid-side to a generator transformer.

Accordingly, the invention simplifies the erection of a pumped-storagepower plant, in particular in the case of an underground construction,for example in a rock cavern, by sparing a transformer between machineand converter. Furthermore, active power losses of the omittedtransformer are saved and the electric unit has a higher efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, details and advantages of the invention result fromthe wording of the claims and from the description of exemplaryembodiments on the basis of the figure.

The invention will be explained in more detail on the basis of thefollowing text with reference to preferred exemplary embodiments usingthe figure, in which

FIG. 1 shows a schematic illustration of an electric unit comprising aconverter, an electric synchronous machine and a generator transformer.

The reference symbols and the significance thereof are summarized in thelist of reference symbols. In general, the same reference symbols denotethe same parts.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a schematic illustration of an electric unit 1 connected toan electric grid 2. The electric unit 1 in this case comprises a modularmultilevel converter 3 and a rotating electric synchronous machine 4.The machine 4 is in this case accommodated in a cavern. In addition agenerator transformer 5 is provided in this embodiment to connect theelectric unit to the electric grid 2.

A frequency conversion is produced by means of a combination of arectifier and an inverter, which are connected to one another via aconcentrated or distributed voltage DC link or current DC link. The DClink in this case furthermore has units for energy storage, for examplecapacitors in the case of a voltage DC link and inductances in the caseof a current DC link.

The operation of the machine at a freely selectable speed hasconsiderable advantages. In particular in the embodiment with afrequency converter and a synchronous machine, an established, reliableand low-maintenance generator technology can be used. Furthermore, thereis the possibility of operating a pump and a turbine independently ofone another in the optimum speed range of said pump and turbine. Byvirtue of the use of the synchronous machine 4, high speeds can beachieved for high drops, for example, in particular even at high powers.Furthermore, the speed range which can be achieved during operationcontinuously ranges from zero to the maximum speed and is onlyrestricted by the operational limits of the pump and the turbine. Thepump and the turbine can in principle be combined in one unit, forexample a pump turbine. In particular, there is the possibility ofretrofitting older systems for variable frequency operation withoutreplacing the existing generator. A further advantage consists in veryquick grid coupling and the possibility of generating positive andnegative reactive power in the converter 3, in order that the generatorcan be operated exclusively with active power, as a result of which saidgenerator has a more compact design. Furthermore, owing to the use ofthe converter 3, it is possible to switch over quickly from pump toturbine operation, for example.

LIST OF REFERENCE SYMBOLS

1 Electric unit

2 Electric grid

3 Converter

4 Electric synchronous machine

5 Generator transformer

1. An electric unit for a pumped-storage power plant, connectable to anelectric grid (2) and comprising: a frequency converter; and a rotatingelectric synchronous machine, wherein the synchronous machine isconfigured for connection to a water turbine and a water pump; whereinthe frequency converter is configured as a modular multilevel converter,and the synchronous machine is directly connected to the frequencyconverter; wherein the frequency converter has is configured to providean adjustable output voltage, to the synchronous machine which isidentical to an input voltage of the machine.
 2. The electric unit asclaimed in claim 1, in combination with a pumped-storage power plant,wherein at least the electric synchronous machine is provided in acavern, in a shaft or in a closed area beneath a tailwater basin of thepumped-storage power plant.
 3. The electric unit as claimed in claim 1,wherein the frequency converter comprises: plural series-connected unitcells for adjusting the output voltage by scaling, wherein the unitcells are separately controllable.
 4. The electric unit as claimed inclaim 3, comprising: a transformer, wherein the individual unit cellsare configured to be controllably switched, an edge steepness andamplitude of a pulse generated by switching of the individual unit cellsbetween an output of the frequency converter and the synchronous machineand the transformer being limitable to such an extent that, even withoutadditional filter elements, no increased demands on isolation of thesynchronous machine and the transformer exist.
 5. The electric unit asclaimed in claim 1, wherein the frequency converter is configured suchthat plural a voltage steps will modify an output voltage of thefrequency converter.
 6. The electric unit as claimed in claim 1, whereinthe output voltage will have limited harmonic distortion such that gridconnection conditions will be fulfilled without additional filtering andno differences emerge for a configuration of the synchronous machinewith respect to harmonics when compared to a direct connection to thegrid.
 7. The electric unit as claimed in claim 1, comprising: agenerator transformer which is connected on a grid-side of the electricunit to the frequency converter.
 8. The electric unit as claimed inclaim 2, wherein the frequency converter comprises: pluralseries-connected unit cells for adjusting the output voltage by scaling,wherein the unit cells are separately controllable.
 9. The electric unitas claimed in claim 8, comprising: a transformer, wherein the individualunit cells are configured to be controllably switched, an edge steepnessand amplitude of a pulse generated by switching of the individual unitcells between an output of the frequency converter and the synchronousmachine and the transformer being limitable to such an extent that, evenwithout additional filter elements, no increased demands on isolation ofthe synchronous machine and the transformer exist.
 10. The electric unitas claimed in claim 9, wherein the frequency converter is configuredsuch that plural voltage steps will modify an output voltage of thefrequency converter.
 11. The electric unit as claimed in claim 10,wherein the output voltage will have limited harmonic distortion suchthat grid connection conditions will be fulfilled without additionalfiltering and no differences emerge for a configuration of thesynchronous machine with respect to harmonics when compared to a directconnection to the grid.
 12. The electric unit as claimed in claim 8,comprising: a generator transformer which is connected on a grid-side ofthe electric unit to the frequency converter.